This invention relates to heart-lung machines, and more particularly to a method and apparatus for simulating the natural heartbeat""s pressure pattern in the blood output of the heart-lung machine.
The natural human heart provides the body with a Pulsatile flow of blood corresponding to the filling and emptying (beating) of the various chambers of the heart. The instantaneous blood flow rate varies in a complex cyclical manner from near zero to some maximum rate, with the overall blood flow rate being a time weighted average.
The cardiopulmonary bypass circuits of heart-lung machines used in open-heart surgery typically utilize centrifugal or positive displacement (i.e. roller type) pumps to provide the motive power for circulation of the blood. These pumps provide an essentially constant flow rate of blood through the circuit at all times, the instantaneous rate and the average rate being nearly identical.
Medical studies have suggested that Pulsatile flow, being more physiologically correct than constant flow, may have a beneficial impact on the efficacy of the extracorporeal perfusion. This can result in improved patient outcomes following cardiac bypass surgery.
Various ways have been proposed to mimic in a heart-lung machine the natural Pulsatile flow of the heart, but none of them have so far been satisfactory. The simplest way of providing a pulsed flow is to cyclically clamp and unclamp the inlet or outlet line of the heart-lung machine""s arterial pump. Clamping the pump inlet is not desirable since it can create very high suction pressures in the inlet which can damage the red blood cells, or in some cases even cause cavitation which can potentially release gas bubbles into the blood stream. Further, during the low flow or rest periods, the pump rotors spin on a stagnant volume of fluid, which may result in mechanical trauma to the blood cells. Clamping the pump outlet is not desirable in a centrifugal pump due to this mechanical trauma. Clamping the pump outlet is not desirable in a positive displacement pump since the rapid buildup of pressure in the lines can rupture the connections or tubing, potentially resulting in a catastrophic event.
A more acceptable way of creating Pulsatile flow is to vary the speed of the pump in a cyclical manner. This is easily accomplished electronically by the pump controller. However, the inertia of the spinning elements of the pump tends to render the resulting waveform more sinusoidal than the natural heartbeat waveform and forces the wave period to be longer than the natural period. In addition, the components of the bypass circuit downstream of the pump, such as the oxygenator and arterial filter, also damp the pulses due to their volumetric holdup.
The present invention creates a Pulsatile flow by causing a continously running pump to fill an elastic bladder with blood through a check valve. The bladder is connected to a blood outlet through an intermittently operable outlet valve. The outlet valve is triggered by the expansion and contraction of the elastic bladder and preferably operates with a hysteresis, i.e. it fully opens when the expansion of the bladder exceeds a predetermined volume, and closes fully when the contraction of the bladder reduces the volume below another, substantially smaller, volume.
In a first embodiment of the invention, a mechanical outlet valve with an appropriate toggle mechanism is directly connected to the bladder so as to snap open when the bladder is fully expanded, and to snap closed when it is nearly empty, the elasticity of the bladder driving the blood through the outlet valve. In a second embodiment, the expansion of the bladder trips a switch or sensor which causes a controller to open the outlet valve and actuate a pressure mechanism which mechanically compresses the bladder to eject the blood. Other known mechanisms with a similar action may also be used.